HDLC Overview

High-Level Data Link Control ( HDLC ) is the default encapsulation used by Cisco routers over synchronous serial links. HDLC is an ISO standard data link protocol. It specifies a method to encapsulate data over synchronous serial links using frame characters and checksums.

Cisco routers use a proprietary version of HDLC. Typically, HDLC is used on leased lines between two Cisco devices. If you need to establish a link between a Cisco router and a non-Cisco device, you must use PPP encapsulation instead of HDLC.

The ISO standard HDLC does not support multiple protocols on a single link. However, Cisco's proprietary HDLC adds a field that allows it to support multi-protocol environments.